Concentrations of interleukin 6 (IL-6) in cerebrospinal fluid (CSF) and serum of infants and children with bacterial meningitis were determined and correlations were sought with other indices of inflammation and with outcome. Forty-two patients ages 1 month to 15 years (mean, 2.5 years) were studied. IL-6 activity was detectable (greater than 50 units/ml) in 30 of 36 CSF samples collected at admission from patients with meningitis and in 1 of 23 controls with fever and normal CSF findings. Mean values were 36,000 units/ml (range, 151-156,000). IL-6 activity in CSF persisted during the first 5 days of illness. IL-6 concentrations at admission were not associated with clinical findings, CSF leukocyte, protein and glucose concentrations, serum C-reactive protein concentration and neurologic complications or sequelae. IL-6 was also detected in the serum of 3 of 14 patients with meningitis and in 0 of 7 controls with no infectious disease. The presence of IL-6 was not associated with bacteremia or with duration of fever before admission. The presence of IL-6 in the CSF of pediatric patients with bacterial meningitis is in accordance with available data on other cytokines and suggests their role as mediators of meningeal inflammation.